For other animals without furrows sea cucumbers, for example , the areas are like the ambiculacra, but usually lack holes for the tube feet. The water vasuclar system opening, called a madreporite, lies on a particular interambulacrum. Letters are used to describe parts of echinoderms.
The ambulacrum opposite the madreporite is section A. Moving clockwise, other parts are coded B through E. Sections C and D are termed the bivium while all the others are collectively termed the trivium. Interambulacrum sections are named using the letters of the ambulacra sections they are between e. Barnes, ; Brusca and Brusca, ; Waggoner, Echinoderms are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3-part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults.
Adult pheromones may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones. Barnes, ; Brusca and Brusca, Echinoderms are mainly gonochoristic having separate sexes , with exceptions among the asteroids , holothurians and ophuroids.
Holothurians possess a single gonad, crinoids lack distinct gonads, while asteroids and echinoids have multiple gonads. Echinoderm reproductive strategies vary from free spawning and indirect development to brooding and direct development. Spawning is probably a noctural event. Parental investment ranges from no care after the release of eggs for free spawning to brooding the young. Brooding is found in polar and boreal echinoderms and some deep sea echinoderms, where environments are more difficult for the larvae.
Most radially symmetric animals are sessile, however, echinoderms are able to move. The water vascular system originally functioned for collection and transport of food, but evolved to function for locomotion as well.
The non-centralized nervous system allows echinoderms to sense their environment from all sides. Different groups have different feeding habits. Members of the Crinoidea sit with arms outstretched parallel to the currents and filter feed on passing particles. Most Asteroidea are predators or scavengers, everting their stomach called a cardiac stomach , which secretes digestive enzymes on their prey.
Some asteroids are also suspension feeders. Brittle stars of the Ophiuroidea are predators, deposit feeders, scavengers, and suspension feeders, which feed by outstretching their arms to capture prey. Ophiuroids lack an intestine and anus, and therefore have an incomplete digestive system. The members of Echinoidea are suspension feeders, herbivores, detritivores, and predators. Many have a group of hard plates which retract and grasp like teeth, commonly called Aristotle's lantern.
This allows most sea urchins to graze on algae. Most Holothuroidea are suspension or deposit feeders. Holothurians may also eviserate their digestive and other organs in response to predation or seasonal events. Echinoderms in general are most vulnerable in their larval stage. As adults, asteroids have an anti-predator adaptation where they can lose an arm to a predator and the arm is later regenerated.
Holothurians discharge sticky tubules, known as Cuvierian tubules , at a potential predator. Otters prey mainly on sea urchins. A pedicellaria snaps open if something touches its outer surface; it snaps shut if it is touched on its inner surface. Some pedicellariae are toxic, containing a small poison gland. Others have powerful jaws that can crush small organisms. The soft inner organs of sea urchins are protected by a hard structure called a test. An urchin test is a hard internal skeleton composed of calcium carbonate CaCO 3 plates Fig.
The plates interlock in a tight geometric pattern that makes the skeleton rigid. Because the test is covered by very thin skin or epidermis, it is considered to be an endoskeleton.
Most of the plates have tiny pores through which internal organs of respiration protrude into the seawater. The spines attach to the plates on tubercles , ball-and-socket joints with muscles attached around the base that support and move the spines. Most sea urchins are herbivores and scrape algae from hard substrates with five tooth-like structures in the mouth on the lower surface of the body. Small bits of food move through a long digestive tube to be digested and absorbed. Indigestible material passes out through the anus, opposite the mouth Figs.
Sea stars belong to the class Asteroidea from the Greek word asteroid meaning like a star ; Fig. Like sea urchins, sea stars inhabit the oceans worldwide, from nearshore tide pools to deep ocean seafloors. Sea stars come in a range of sizes, reaching up to one meter m in length, but most are much smaller. Sea stars may be red, blue, or many other colors. Most sea stars have a central disk with five radial arms; some species have 15 to 40 arms. A few species have arms so short that they barely protrude.
Their bodies look like pin cushions. These plates, called ossicles , are much smaller than those of sea urchins. Some sea stars have spines extending from the ossicles, to help defend them from predators. Sea stars have a water vascular system and tube feet much like those of the sea urchins. Ambulacral grooves from the Latin root ambul meaning walk are narrow channels in the oral surface of a sea star filled with tube feet.
The tube feet are used mainly for grabbing and locomotion. Sea stars have remarkable powers of regeneration. Many species can regenerate a whole arm that breaks off Fig. These regenerated pieces are called comets Fig.
Sea stars are voracious predators, crawling over the ocean bottom in search of prey. They feed not only on sessile molluscs such as clams, oysters, and mussels, but also on dead organisms lying on the bottom.
The crown-of-thorns starfish Acanthaster planci consumes so much live coral that it is considered a significant threat to coral reefs in the tropical Indo-Pacific region. The mouth of a sea star opens into the stomach in the central disc.
The anus is on the upper surface Fig. Most sea stars are carnivores. Although a sea star has no teeth, it can eat coral polyps and molluscs by pushing its stomach out of the body, spreading it over its prey, and digesting it. To eat a clam, the sea star grasps the bivalve in its arms, attaches its suction cups to both shells, pulls steadily until the shells open slightly, and extends its stomach into the clam. In this way it preys on clams whose shells are open as little as 0.
After the sea star digests and absorbs the tissue of its prey, it sucks its stomach back into its own body. Brittle stars are the most abundant echinoderms. About 2, species inhabit the ocean floor worldwide, from the shoreline to great depths. In some areas, clusters of millions of brittle stars thickly carpet the bottom. This group is active only at night, hiding under rocks and in crevices during the day. Brittle stars have long, flexible arms attached to a small central disc Figs.
Skeletal ossicles form a series of scaly plates along the arms, and a series of large cylindrical ossicles runs through the center of each arm. These ossicles look somewhat like the row of vertebrae in a fish skeleton. They are connected by muscles that contract, producing a snakelike action.
This characteristic movement gives the class its name, Ophiuroidea from the Greek root words ophio - meaning snake and - uroid meaning tail-like. It is also the basis for another common name, serpent star. Moving brittle stars can appear to be dangerous, but they are harmless to humans.
A row of movable spines projecting from the sides of the arms helps the animal move along the bottom. Although the arms appear to be radial, one or two of them usually lead in pulling the animal along while the others trail Fig. These animals got the name brittle star because an arm often breaks off if they are captured.
The broken arm is left wiggling as the rest of the brittle star scoots away. The missing arm regenerates quickly. Most brittle stars are small with a central disc diameter less than three centimeters, but the arms may be up to ten centimeters long.
Brittle stars feed on detritus—small particles of food—on the bottom. Some brittle stars curve their arms up to collect food particles suspended in the water. The tube feet, shaped like pointed tentacles, are used mainly for collecting food. One tube foot passes particles to another toward the mouth. The food then passes into the stomach, where it is digested.
Unlike the other echinoderms, brittle stars have no anus; they eject undigested material through the mouth. The class Holothruoidea is better known by as the sea cucumbers. Sea cucumbers are cylindrical echinoderm animals with feathery tentacles at the mouth end of their bodies. They are often mistakenly called worms. Some species resembles fat pickles a few centimeters long Fig. Others are like thin tubes over a meter long Fig. These animals are common residents of reefs and rocky shorelines worldwide.
By using hydrostatic pressure, the animal can either protrude or retract the tube feet. Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal.
The ring canal connects the radial canals there are five in a pentaradial animal , and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals. Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms.
These podocytes are connected by an internal system of canals to an opening called the madreporite. Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma. This phylum is divided into five extant classes: Asteroidea sea stars , Ophiuroidea brittle stars , Echinoidea sea urchins and sand dollars , Crinoidea sea lilies or feather stars , and Holothuroidea sea cucumbers Figure 2.
Figure 2. Different members of Echinodermata include the a sea star of class Asteroidea, b the brittle star of class Ophiuroidea, c the sea urchins of class Echinoidea, d the sea lilies belonging to class Crinoidea, and d sea cucumbers, representing class Holothuroidea.
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety of shapes, colors, and sizes, with more than 1, species known so far.
The key characteristic of sea stars that distinguishes them from other echinoderm classes includes thick arms ambulacra that extend from a central disk where organs penetrate into the arms.
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